Soil Amendment

The reason for soil amendment is to provide a better environment for roots and plant growth: this includes the improvement of the soil structure and water holding capacity, the availability of nutrients, and the living conditions for soil organisms, which are important for the plants to grow. Furthermore, a better soil texture and better root growth avoids soil degradation during heavy rains or in windy regions. It also supports the nutrient cycle when organic amendments are used (e.g. manure). Beside soil amendment, there are several methods to provide soil moisture conservation such as soil cover and reforestation (living plants), mulching or several tillage techniques. Of course it is also very important that a crop is planted which is suitable for the given climate. Basically, any organic or inorganic material that is added to the soil and improves its quality can be considered as soil amendment. The type of amendment chosen depends entirely on how the soil needs to be changed. By using soil amendment, almost every type of soil can be made fertile (WEST COAST SEEDS 2011).

Both  inorganic and organic substances can be added into the soil. Organic substances consist of material derived from living things (e.g. plants), whereas inorganic substances are mined or man-made.

Inorganic Amendments

Inorganic substances are vermiculite, perlite, tire chunks, pea gravel and sand. In general, they must be bought, which makes them more expensive than organic amendments. Additionally, the industrial production process needs a great deal of energy. Therefore, these substances do not have the same degree of sustainability as organic amendments. Most are relatively sterile (with regard to plant pathogens) and many are relatively inert. Inorganic amendments are used to (REED 2007):

• increase aeration
• increase drainage
• decrease excessive water holding capacity
• decrease or increase weight.

Organic Amendments

Organic growing medium amendments usually are derived from plants or plant products that occur naturally (peat moss from peat bogs), or are the by-products of processing plants or mills (sawdust, cedar chips, bark, bagasse, rice hulls) or waste disposal plants (compost, processed sewage sludge, biosolids). The main purpose of using organic amendments is to loosen the soil and create large pores to increase (REED 2007):

• aeration
• drainage
• usable water holding capacity
• nutrient holding capacity
• decrease growing medium weight (compared to soil)

By far, the most important organic soil amendments worldwide are compost and animal dung (e.g. chicken, cow). Compost and animal dung are most often easily available and economically affordable.

In professional agriculture, the most commonly used organic amendment is peat moss (e.g. sphagnum peat moss which is the highest quality, but also hypanaceous or domestic peat). Other organic amendments include wood chips, grass clippings, straw, compost in general (produced in grande or small scale facilities), manure (see also use of compost, use of dehydrated faeces or terra preta), processed sewage sludge (biosolids), sawdust, etc. Normally, there are more organic substances available than inorganic substances and generally they are cheaper (REED 2007; GIZ 2010). Practising good crop rotations and choosing the correct crop improves soil amendment.

Before incorporating into ground beds, most organic amendments, especially sawdust, cedar chips, bark and bagasse, should be composted or aged, and sterilised before use if possible. All of these amendments have a high C:N ratio. Use of amendments which are not composted and have a high C:N ratio will deplete N from the soil, may cause a salt or an ammonia/ammonium burn, or may cause damage due to heat build up.

Amendments with a low C:N ratio will release N upon further decomposition, thus act as an organic fertiliser. However, organic matter with a low C:N ratio also should be composed to avoid rapid ammonia/ammonium release and toxicity (adapted from REED 2007).

Advantages of Organic Amendments

Humic acid is a principal component of humic substances, which are the major organic constituent of organic soil amendments. Organic soil amendments (humus) have been known by farmers to be beneficial to plant growth for longer than recorded history. It was supposed that humus was used directly by plants, but it was shown later that plant growth depends upon inorganic compounds. Therefore, many soil scientists held the view that organic matter was useful for fertility only as it was broken down with the release of its constituent nutrient elements into inorganic forms. At the present time, soil scientists hold a more holistic view and at least recognise that humus influences soil fertility through its effect on the water-holding capacity of the soil: the spongy structure of organic matter is able to bind water and some inorganic molecules which act as micro- or macro-nutrients. Consequently, humic acids also slow water evaporation from soils. This is especially important in soils where clay is not present or in a low concentration, in arid areas, and in sandy soils without the capability to hold water. The oxygen end of another water molecule bonds with the hydrogen end of another, until the evaporation rate is reduced by 30% (NUTRANETICS 2000).

An additional benefit of organic amendments is also the fact that organic matter feeds soil microbes, which in turn release nutrients into the soil, thereby increasing soil fertility.

Factors to Consider When Choosing an Amendment

(Adapted from DAVIS & WILSON 2005)

There are at least four factors to consider in selecting a soil amendment:

• How long the amendment will last in the soil
• Soil texture
• Soil salinity and plant sensitivities to salts
• Salt content and pH of the amendment

Laboratory tests can determine the salt content, pH and organic matter of organic amendments. The quality of bulk organic amendments for large-scale landscape uses can then be determined. The amendment you choose depends on your goals.

• Are you trying to improve soil physical properties quickly? Choose an amendment that decomposes rapidly.
• Do you want a long-lasting improvement to your soil? Choose an amendment that decomposes slowly.
• Do you want a quick improvement that lasts a long time? Choose a combination of amendments.

Table 1: Permeability and water retention of various soil types. This can be changed by adding an amendment (see table 2). Source: DAVIS & WILSON (2005)

Table 2: Depending on crops, permeability and water retention of soils (table 1) can be adjusted. Source: Davis & Wilson (2005)

Most inorganic amendments are mined from natural deposits and further processed to yield the final product. Heavy machinery and technical know-how is necessary to produce such amendments. These facts cause higher costs in production. In the other hand, organic amendments have high self-help compatibility, because farmers can produce them on-site. There are numerous methods that help to produce organic amendments and support a sustainable handling of the local nutrient and water cycle. Composting toilets, terra preta toilets, fossa alternas or arborloos, UDDT are toilet systems that generate organic amendments. According to GIZ (2010), there are more organic substances available than inorganic, and they are generally cheaper.

Environmental Effects

Soil amendments have to be handled carefully. Soil does not have the capacity to bind substances ad infinitum without releasing them. If too many nutrients or organic matter are put into the soil, they can be released and cause an outflow into the groundwater and surrounding rivers and lakes, which can cause water pollution (GIZ 2010).